The present invention relates to implantable cardiac pacemakers, and particularly to pacemakers capable of providing rate-responsive pacing therapy. More particularly, this invention is directed toward an input protection circuit for pacemaker activity sensor processing circuits that does not produce undesirable offset voltages.
A pacemaker is an implantable medical device which delivers electrical stimulation pulses to cardiac tissue to relieve symptoms associated with bradycardia--a condition in which a patient cannot normally maintain a physiologically acceptable heart rate. Early pacemakers delivered stimulation pulses at regular intervals in order to maintain a predetermined heart rate, typically a rate deemed to be appropriate for the patient at rest. The predetermined rate was usually set at the time the pacemaker was implanted, although in more advanced pacemakers, the rate could be set remotely after implantation.
Early advances in pacemaker technology included the ability to sense the patient's natural cardiac rhythm (i.e., the patient's intracardiac electrogram, or "IEGM"). This led to the development of "demand pacemakers"--so named because they deliver stimulation pulses only as needed by the heart. Demand pacemakers are capable of detecting a spontaneous, hemodynamically effective cardiac contraction which occurs within a predetermined time period (commonly referred to as the "escape interval") following a preceding contraction. When a naturally occurring contraction is detected within the escape interval, the demand pacemaker does not deliver a pacing pulse. The ability of demand pacemakers to avoid delivery of unnecessary stimulation pulses is desirable because by doing so, battery life is extended.
Demand pacemakers allow physicians to telemetrically adjust the length of the escape interval, which has the effect of altering the heart rate maintained by the device. However, in early devices, this flexibility only allowed for adjustments to a fixed programmed rate, and did not accommodate patients who required increased or decreased heart rates to meet changing physiological requirements during periods of elevated or reduced physical activity. Therefore, unlike a person with a properly functioning heart, a patient receiving therapy from an early demand pacemaker was paced at a constant heart rate, regardless of the level to which the patient was engaged in physical activity. Thus, during periods of elevated physical activity, the patient was subject to adverse physiological consequences, including light-headedness and episodes of fainting, because the heart rate was forced by the pacemaker to remain constant.
More recently, pacemakers have become available that are capable of adjusting the rate at which pacing pulses are delivered in accordance with a patient's metabolic needs. These devices, known as "rate-responsive pacemakers," typically maintain a predetermined minimum heart rate when the patient is engaged in physical activity at or below a threshold level, and gradually increase the maintained heart rate in accordance with increases in physical activity until a maximum rate is reached. Rate-responsive pacemakers typically include processing and control circuitry that correlates measured physical activity to a desirable heart rate. In many rate-responsive pacemakers, the minimum heart rate, the maximum heart rate, and the transition rates between the minimum heart rate and the maximum heart rate are parameters that may be adjusted to meet the needs of a particular patient.
One approach that has been considered for enabling rate-responsive pacemakers to determine an appropriate heart rate involves measuring a physiological parameter that reflects the level to which the patient is engaged in physical activity. Physiological parameters that have been considered include central venous blood temperature, blood pH level, QT time interval and respiration rate. However, certain drawbacks (such as slow response time, unpredictable emotionally-induced variations, and wide variability across individuals) render the use of these physiological parameters difficult, and accordingly, they have not been widely used in practice.
Rather, most rate-responsive pacemakers employ sensors that transduce mechanical forces associated with physical activity. These activity sensors generally contain a piezoelectric transducing element which generates a measurable electrical potential when a mechanical stress resulting from physical activity is experienced by the sensor. By analyzing the signal from a piezoelectric activity sensor, a rate-responsive pacemaker can determine how frequently pacing pulses should be applied to the patient's heart.
Piezoelectric elements for activity sensors are commonly formed from piezoelectric ceramics, such as quartz or barium titanate. Recently, however, activity sensors have been designed which use thin films of a piezoelectric polymer, such as polyvinylidene fluoride (commonly known by the trademark KYNAR, owned by ATOCHEM North America) as the transducing element, rather than the more commonly used piezoelectric ceramics. Activity sensors so designed are described in, commonly assigned U.S. Pat. No. 5,383,473, entitled "Rate-Responsive Implantable Stimulation Device Having a Miniature Hybrid-Mountable Accelerometer-Based Sensor and Method of Fabrication," and U.S. Pat. No. 5,425,750, entitled "Accelerometer-Based Multi-Axis Physical Activity Sensor for a Rate-Responsive Pacemaker and Method of Fabrication," which are hereby incorporated by reference in their entireties.
The activity sensors described in the above-incorporated patent applications offer significant advantages over sensors which use piezoelectric ceramics. These advantages are largely attributable to the resiliency of the thin polymer films. For example, the piezoelectric polymer films are better able to withstand stresses that may occur during sensor fabrication, thereby reducing the cost and complexity of the fabrication process. In addition, activity sensors which use the polymer films may be designed to respond more aggressively to mechanical stresses resulting from physical activity, so that they provide stronger output signals. Indeed, the output potentials provided by activity sensors that use polyvinylidene fluoride transducing elements typically have magnitudes of about 200 mV (RMS), whereas piezoelectric ceramic sensors provide output potentials which typically have magnitudes of just a few mV (RMS).
Despite the widespread use of piezoelectric sensors in rate-responsive pacemakers for measuring physical activity, certain difficulties remain which have yet to be overcome. One difficulty relates to the conventional use of a pair of diodes as an input protection circuit for the processing and control circuitry. Although such an arrangement effectively protects the processing and control circuitry from electrostatic discharge potentials, as well as abnormally high signals generated by the piezoelectric sensor (as may occur, for example, if the sensor is mishandled during fabrication), the conventional input protection circuit has an undesirable side effect. Specifically, during normal pacemaker operation, a small leakage current flows through one of the diodes while that diode is reverse biased. This small leakage current flows through a large resistance in parallel with the processing and control circuitry. Thus, an undesirable DC offset potential appears across the processing and control circuitry--even in the complete absence of patient activity.
Under most circumstances, the undesirable DC offset potential may be about 26.7 mV. This could result in an erroneously high determination of the patient's activity level by the processing and control circuitry. Of further concern, however, is that the leakage current through the reverse biased diode is highly temperature dependent--doubling about every 9.degree. C. Since the pacemaker is typically implanted just below the patient's skin, the temperature of the pacemaker can vary with the temperature of the external environment. Thus, the magnitude of the offset potential can vary considerably as the leakage current varies with temperature, making the task of accurately processing the sensor signals even more difficult.
What is needed therefore is an improved input protection circuit that allows the processing and control circuitry of a rate-responsive pacemaker to process activity sensor signals more accurately. The improved input protection circuit should be manufacturable in an efficient and cost-effective manner, and it should not have an adverse impact on the size or operation of the pacemaker.